RFR: 8308583: SIGSEGV in GraphKit::gen_checkcast

[Roland Westrelin]

At an `instanceof`, a node of type `bottom[int:>=0]` is checked to be of type `cc$Word` and on the success a `CheckCastPP` is inserted to change the node type. That `CheckCastPP` constant folds to `top` but the type check doesn't fold. The reason is that the type check loads the klass from the node with a `LoadNKlass` and the type of that node is `java/lang/Object`  when it should be `bottom[int:>=0]` but logic in `LoadNKlass::Value()` gets in the way.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jdk/pull/14123/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14123&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308583
  Stats: 103 lines in 3 files changed: 101 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14123.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14123/head:pull/14123

PR: https://git.openjdk.org/jdk/pull/14123